rust: silence unknown warnings for the sake of old compilers

Occasionally, we may need to silence warnings and clippy lints that
were only introduced in newer Rust compiler versions.  However, this
would fail when compiling with an older rustc:

error: unknown lint: `non_local_definitions`
   --> rust/qemu-api/rust-qemu-api-tests.p/structured/offset_of.rs:79:17

So by default we need to block the unknown_lints warning.  To avoid
misspelled lints or other similar issues, re-enable it in the CI job
that uses nightly rust.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

4 files changed, 15 insertions, 1 deletions

diff --git a/.gitlab-ci.d/buildtest.yml b/.gitlab-ci.d/buildtest.yml
index 4800aed04b..336223484d 100644
--- a/.gitlab-ci.d/buildtest.yml
+++ b/.gitlab-ci.d/buildtest.yml
@@ -128,7 +128,7 @@ build-system-fedora-rust-nightly:
     job: amd64-fedora-rust-nightly-container
   variables:
     IMAGE: fedora-rust-nightly
-    CONFIGURE_ARGS: --disable-docs --enable-rust
+    CONFIGURE_ARGS: --disable-docs --enable-rust --enable-strict-rust-lints
     TARGETS: aarch64-softmmu
     MAKE_CHECK_ARGS: check-build
   allow_failure: true
diff --git a/meson.build b/meson.build
index 8ee4b7dd15..01d0d542f2 100644
--- a/meson.build
+++ b/meson.build
@@ -3346,6 +3346,14 @@ if have_rust and have_system
   # Prohibit code that is forbidden in Rust 2024
   rustc_args += ['-D', 'unsafe_op_in_unsafe_fn']
 
+  # Occasionally, we may need to silence warnings and clippy lints that
+  # were only introduced in newer Rust compiler versions.  Do not croak
+  # in that case; a CI job with rust_strict_lints == true ensures that
+  # we do not have misspelled allow() attributes.
+  if not get_option('strict_rust_lints')
+    rustc_args += ['-A', 'unknown_lints']
+  endif
+
   # Apart from procedural macros, our Rust executables will often link
   # with C code, so include all the libraries that C code needs.  This
   # is safe; https://github.com/rust-lang/rust/pull/54675 says that
diff --git a/meson_options.txt b/meson_options.txt
index 24bf009056..502efe605b 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -378,3 +378,5 @@ option('x86_version', type : 'combo', choices : ['0', '1', '2', '3', '4'], value
 
 option('rust', type: 'feature', value: 'disabled',
        description: 'Rust support')
+option('strict_rust_lints', type: 'boolean', value: false,
+       description: 'Enable stricter set of Rust warnings')
diff --git a/scripts/meson-buildoptions.sh b/scripts/meson-buildoptions.sh
index 6f2bb08ecd..f9693d226b 100644
--- a/scripts/meson-buildoptions.sh
+++ b/scripts/meson-buildoptions.sh
@@ -47,6 +47,8 @@ meson_options_help() {
   printf "%s\n" '                           getrandom()'
   printf "%s\n" '  --enable-safe-stack      SafeStack Stack Smash Protection (requires'
   printf "%s\n" '                           clang/llvm and coroutine backend ucontext)'
+  printf "%s\n" '  --enable-strict-rust-lints'
+  printf "%s\n" '                           Enable stricter set of Rust warnings'
   printf "%s\n" '  --enable-strip           Strip targets on install'
   printf "%s\n" '  --enable-tcg-interpreter TCG with bytecode interpreter (slow)'
   printf "%s\n" '  --enable-trace-backends=CHOICES'
@@ -493,6 +495,8 @@ _meson_option_parse() {
     --disable-spice-protocol) printf "%s" -Dspice_protocol=disabled ;;
     --enable-stack-protector) printf "%s" -Dstack_protector=enabled ;;
     --disable-stack-protector) printf "%s" -Dstack_protector=disabled ;;
+    --enable-strict-rust-lints) printf "%s" -Dstrict_rust_lints=true ;;
+    --disable-strict-rust-lints) printf "%s" -Dstrict_rust_lints=false ;;
     --enable-strip) printf "%s" -Dstrip=true ;;
     --disable-strip) printf "%s" -Dstrip=false ;;
     --sysconfdir=*) quote_sh "-Dsysconfdir=$2" ;;